Multiplex carrier-wave communication



April 16, 1929. E. H. sM'Y'rHE IULTIPLEX CARRIER WAVE COMUUNICATION April 16, 1929. E H, SMYTHE 1,709,044

MULTIPLEX CARRIER WAVE COMMUNICATIONv Filed Dec. 23. 1922 8 Sheets-Sheet 5 //4 Tc 42 lsrc 45 Aff?.

April 16, 1929. E. H. sMY-rHE KULTIPLEX CARRIER WAVE CHHUNICATON Filed Dec. 2s'. 1922- s sheets-'sheet 4 April 16, 1929.

E. H. sMYTHE VIULTIPLEX CARRIER WAVE COMMUNICATION Filed Dec. 23. 1922 8 Sheets-Sheet 5 w r. ,m m M 5 y J//. im mw .t #bm .n.m mm u@ April 1e, 1929.

E. H.A SMYTHE MULTIPLE); CARRIER .wAvE couuuulcurn 6 .M m ny.

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Filed Dec. 2 3 l1.922

April 16, 1929. E. H. sMYTHE 1,709,044

T IULTIPLEX CARRIER WAVE COMMUNICATION Filed Dec. 23, 19224 8 Sheets-Sheet 'i April 16, 1929. E. H. sMYTHE IULTIPLEX CARRIER WAVE COHKUNICATION Filed nec. 2s. 1922 a sheets-sheet' s W MW m. ww 96%- ,.Qkwb MQ Y Si@ NQ Mt* E S@ HIL w el t) latented Apr. 16', 1929.

Lanai nieren sra'iss PATENT oFFice.

EDWIN ITI. SMYTHE, OF EVANS'EN, LLENES, ASElTill-lllt TO Tl/STERN ELECTRC COM- PANY, INCRPORATED, GIF NEW' YORK, N. Y., i CORFOR-A'IION OF NEW YORK.

llv'ltl'L'lIPl'ilX CARRIER-WAVE COMMUNICATION.

Application inea necemiier 23, 1922. seran No. coarse.

This invention relates to multiplex carrier Wave communication7 and more particularly to radio toll linlrs lier connecting' ordinary telephone and telegraph systems ttor two-Way communication.

Carrier Wave transmission is accomplished by means o'i'i high frequency oscillations `termed carrier Waves which are modified or modulated in accordance with the signals to be transmitted, lllhei'e tivo ordinary conductive communication circuits are to he connected by meanso't a carrier Wave link ttor simultaneous tivo-way tiiiiisniissioii, nach ot the conductive communication circuits is operatively associated with carrier transi'nittingl and carrier receiving apparatus. rlllhe carrier transmitting,I apparatus serves to modulate a carrier wave in accordance with the speech or signal to be transmitted and to transmit the energy ot the modulated Wave over a carrier circuit or to radiate it into space. The carrier receiving apparatus selectively receives carrier Waves modulated at distant station and deinodnlatcs the received Waves to produce lovv 'trequency currents corresponding to the loiv .frequency speech or sign al currents by Which these carrier Waves were modulated at the distant station. Where a plurality oit communications are to be etlected simultaneouslyj the carrier Waves are assigned such 'frequencies as to be noneinterering one with another; or in other Words each et a plurality oit cai rier were transniittiiic; apparatus is so orgaan `ir/.ed as to produce, and each corresponding carrier wave receiving' apparatus is so ork d as to receiveq a pi "ticular `frequency et carrier Wave to the exclusion ot carrier Waves Vproduced and received by the other trai liitting and receiving,l apparatus. ln a syste-in 'tlius organized there may be as many siinultaiienously proceedingi` and noiuinter- 'lei-in communications as there are non-iii tert'eriiigcarrier Waves with apparatus torv transmit eeivine` by iiieans of a wave oil qiiei1cy,`is comin-only known as achannel, or

a higili 'tiropeney eininunication channel.

ln the iii'z vii 'ion the selective prei orties o1" high trequency or carrier Wave producing, transmitting and receiving,` devices are utilized to provide a system tor operatively relatiii` a plurality of toll centers or stations through the medium of carrier wave links in such a way that there may be estal1- lished between these stations a plurality olf simultaneous and nonintertering intercommunications without any restriction as to whether the connections which may simultaneously exist with each station are incoming or outgoing with respect to that station. Iii other words the system is so organized that each station inay be the calling stationivith respect to certain of the conections and at the saine time the called stationivith respect to others ot the connections that simultaneously exist; and each station may employ .its entire radio or high frequency trunliing 'facilities simultaneously in either calling;K one or more olf' the other stations, or in bein called liv one or more et the other stat-ions, or in callino confie and beine' called by others, in ivliuteveix way tra'llic conditions may happen to require and Without interlfercnce et one coii'in'iunication with another, This may be, and in the present embodiment oi the invention is, ac-

complished by means el a single pair of antennae at each station.

l ln the particular en'ibodiinent et the invention to be specifically described hcreinaiter, tour carrier Wave .stations 'will be assumed to comprise thesystein, but it is to be understood that the systeni `may have any desired number ol. stations and that these stations inay be provided lay merely duplicating; and entendingl the arraiin'einents ollthe tour sin tions der-scribed.

ln the present eii'ihodiinent et the invention each radio toll center or station is assigned a certain number oil carrier Waves entre quencies tor 'transmitting purposes; and each station has i f'ailablc for use as many receivini;l channels, preferably permanently or ganizec as there are carrier ivaves adapted to Y xie tninsmitted by the other distantstations collectively. Each ol' these receiving` channels at each station is so organised as tobe selectively responsive to a dill'erent one oit the carrier .vaves lthat the other distant stations are arranged to transn'iit. Ait none et the stations are there receiving channels responsive to the particular carrier Waves transmitted from that station; and further to prevent the locally transmitted 'frequencies 'trein interfering with the corresponding receiving system,

the receiving antenna system at each station includes a device, which may be in the form of aV band filter, that acts to divert the ieceived energy of the locally transmitted frequencies from the receiving branches of the receiving antenna system and to dissipate this energy, if desired, in suitable resistance elements or an artificial line.

Of the carrier wave transmitting channels with which each station is provided, one serves, inthe present embodiment of the invention,- as a channel for transmitting a wave which is modulated tol control the signals incident to the establishment and supervision of the connections, while the other transmittingchannels are employed to transmit the waves that vare modulated to convey the speech or communicating currents. At each station the group of communication transmitting channels and the several groups of communication receiving channels, one group for receiving the carriers from each rofthe distant stations, are conveniently terminated in springjacks with which a number of plug-terminated connecting cord circuits cooperate to associate, as desired, a transmitting channel with a receiving channel to constitute Va radio link for two-way communication, and to connect the two-way radio trunk thus formed with any desired one of a number of springjack terminated locallines. f

As each vof the stations of the system has certain frequencies Awhich-V it employs in transmitting, the receipt ofthe frequency assigned to a lparticular station definitely identifies that station as the calling station;

Vand the modulation of the calling stations controlling or calling` frequency by lower frequencies in various combinations serves toidentify theA particular onerof the distant stations that is being calledk and the particular one of its communicating channels that lthe calling station has selected for establishing the connection. The combinationY of lower frequencies which the calling station employs to modulate its calling or controlling carrier wave causes the display at the particular station that is being called of a rsignal associated with the particular communication receiving channel corresponding tothe transmitting communication channel which the calling station has selected for use. VThe signals associated with the correspondving receiving channels at the stations other than the one being called are not displayed. The establishment of the two-way radio link between the calling and called stations is completed by the called station selecting and Vconnectifngwith one of its available Vcommunicating channels, and indicating the selected channel to the calling station by modulating its own calling or controlling carrier wave with the proper combination of lower frequencies to cause the dis lay of the corresponding signal at the calling station.

In the present embodiment of the invention two base frequencies generated in any convenient manner and transmitted from any convenient point aie received at each of the stations; and harmonics generated from these base frequencies are combined to produce carrier waves for modulation by speech or controlling frequencies in the transmitting channels of each station, and for the demodulation of the modulated carrier waves in the receiving channels at each station. The combination of the base frequencies at each station is effected in such a manner that each of the combining units serves to produce a number of the desired carrier and deinodulating waves as a plurality of readily separable side bands of the combined harmonics of the base frequencies.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and mode of operation, together with the particular objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawings, which constitute a disclosure of the preferred embodiment of the invention. In the accompanying drawings, Fig. 1 illustrates in schematic form a carrier wave communicating system comprising four stations organized for general intercommunication through the medium of a plurality of non-interfering carrier Waves; Fig. 2 illustrates schematically, but in greater detail, the organization of one of the stations, station 1, of Fig. 1; and Figs. 3 to 8, inclusive, when arranged as indicated in Fig. 9, show in detail as much of the circuits of one of the stations as is necessary to enable the wiring and organization of the entire system to be understood.

Referring first to Fig. 1, this schematic drawing is merely intended to show an outline or skeleton of the general arrangement and relationship of four stations adapted for interconnection through the medium of high frequency or carrier waves in accordance with the plan of the present invention. These stations are indicated in the present instance as radio stations, that is, stations between which interconnection and intercommunication are established through the medium of unguided high frequency waves transmitted through space. In the specification that follows, the high frequency stations will occasionally be referred to as radio stations but it is to be understood that the use of this term is not intended to limit the invention to exclude a systein the stations ofi which are operatively rello llfi

llitl 1,709,044. til

waves guided upon conductors, as the stations he of this character. Furthermore, in Fig. l as well as in the other iigures of the drawing, definite frequency values are indi cated as being assigned to the carrier waves, to the base frequencies and to the lower con trolling frequencies with which the calling or controlling carrier wave of each station is adapted to be modulated. It should be un derstood, however, that these definite and spe- Vcii'ic frequency values are used merely for the purpose of clarity in the disclosure and that a wide diversity of frequency values may be substituted, as desired, for the purely typical and illustrative values that are herein employed Awithout in any way departing from the spirit of the invention. i

ln the typical arrangement indicated in outline in Fig. 1 there are four stations, station l, station 2, station 3 and stat-ion 4, respectively; and in addition thereto there is illustrated a transmitting station designated Ease frequency source which may be assumed to be any convenient location,at some separate point, at any one of the four stations indicated, or, if desired, may have its function served by the transmitting antenna of one of the four stations. Associated with the base transmitting antenna are oscillation generators of any suitable type which act to produce two base frequencies, in the present instance `100,000 and 107 ,000 cycles per second, and to transmit these base frequencies through the base transmitting antenna to the other stations of the system. The frequencies that are employed in the system and which the stations use for interconnection are indicated in l hy a numher of broken lines. The lowest of these broken lines is designated 100,000 and 107,000 cycles. The other fourbroken lines are designated respectively 505,000 to 500,000, 614,000 t@ 635,000, 665,000@ 686,000 and 714,- 000 to'?35,000. The broken line connection of the transmitting antenna of each of the four stations to a particular one of the four last mentioned hrolren lines indicates that the corresponding transmitting antenna is adapted to utilize for transmitting purposes the par ticular band of frequencies of the broken-line with which it has connection. Thus station l is represented as transmitting on the 505,000 to 580,000 band, station 2 on the 614,000 to 005,000 band, station 3 on the 605,000 to 000,- 000 band, and station el on the ll-1,000 to 735,- 000 band.

The broken line connections of the reeeiv- 'ing antenna at each station with the horiof the organization disclosed may 572,000 cycles respectively for eoinmunicating and a modulated wave of 505,000 cycles for signal controlling; station 2 tranrnnits carrier waves of lila-,000, 021,000 and 020,000 cycles respectively for communicating and a wave of 035,000 cycles for controlling; station 3 transmits waves of 600,000, 079,000 and 072,000 cycles respectively for communicating and a wave of 005,000 cycles for controlling and station l transmits waves oi 714,000, 72l,000 and r720,000 cycles respectively for cmninunieating and a wave of 735,000 for controlling: '.ihe receiving antenna system of each station in the embodiment of the invention illustrated, has a tuned branch responsive to each of the four carrier wave frequencies transmitted by each ofthe other three or distant stations, as well as a tuned branch responsive to the two hase frcquencies of 100,000 and 107,000 cycles. There is also included in the circuit of the receiving` antenna. of each station. `a'tuned branch re sponsive to the frequencies of the associated transmitting antenna, which branch, as will be explained in connection with Fig. 5, acts as a trap to divert the energy received from the local transmittingantenna and largely prevent it passing throiilgh the portion of the circuit including the other tuned hranches of the receiving antenna system.

The general scheme of organization of the various stations of the sysfem will now he explained by reference to Fig. 2 of the draw ing. The {nirticular ation that has been chosen for sehr-anali .lustration iu this lic.'- ure of the dran' ,Y is station l. lt will he understood that the plan of each of the other stations is the saine excepting for such differences as have already heen indicated in the matter of the particular frequen-:z' assigned to the diliierent transmitting` and relceiving channels. .llor the purpose of sinlnlified and concise represi-intation, inestof lthe devices and groups of apparatus that constitute elements i', the organization are indica ed in Fig. Q by means of appropriatelv lettered squares. For convenient reference na ley to the lettered srpiares in this and the other 'ligures is `here given. rlhe squares niarlred .A are anlpliliers. The squares ma rated lili are balanced ampli tiers. Squares ll-ll are hand filters. `ffiquare lll., is an artiiicial line or energy dissiiiating` device. Squares-i lill) are balanced detectors.n Squares TC are tuned circuits. Squares STC are Vr621,000, 628,000 and 635,000 cycles.

tuned circuits with .anti-resonant shunts.

Squares LPFV are low pass filters.y Squares M are modulators. Squares D are detectors. Squares HG are harmonie generators. Squares O are oscillators. Squares BM are .balanced modulators. Squares H i/ are hybrid or conjugate coils. Squares N are balancing networks. VIn addition a nuiiiher ot Aiiiilettered squares are employed which inercers and transmitters, etc., are represented by their usual conventional symbols, so that in explaining the plan ot organization illustrated in Fig. 2 .it will be unnecessary to identit'y by reference numerals the elements to' which there may be occasion to reier. The various apparatus referred to above as being indicated by'squares in certain figures ot the drawing will be described more fully hereinafter, andreterence is here made to the paper by Colpitts and Blackwell entitled Carrier current telephony and telegraphy, published in the Journal ot the American Institute of Electrical Engineers, April 1921, and succeedingissues, for a disclosure oi. individual apparatusand subcombimitions ot' apparz'itiis otI the general type employed in the present system.

The receiving antenna system is made aperiodic, or in any convenient and well knownway is given as many degrees ol treedoin as are necessary to make it responsive to the range oit frequencies that are to be received. The output circuit-of the balanced amplifier isconnected with as many timed branches as there are carrier-waves to be received 'from the various distant stations :ind from the base frequency generating station.

V'llliebranclies `for responding to the carrier waves from station 2 are designated channel '1, channel '2, channel 3 and control channel respectively,l these y branches, kas indicated, being responsive respectively to 614%00, lie

rbranches of the receiving antenna system adapted to respond to the carrier vwa ves from station 3 are designated channel 1., channel 2, channel 3 and control channel and are respectively responsive to frequencies ot 686,000, 679,000, 672,000 and 665,000 cycles.

' lThe branches ot the receiving antenna system adapted 'for cooperation with station l are likewisey designated channel 1, channel 2,

vchannel 3 and control channel, and are respectively `responsive to carrier waves ot 714,000, 721,000, 728,000 and 735,000 cycles. The branch of the receiving antenna system responsive to the base frequency waves, which branch is shown at thc lower left hand corner ot Fig. 2 is responsive to a range of frequencies iroin 100,000 to 107,000 cycles inelusive.

Figs. 1 and 2 have been referred to for illustration ot the general layout ot the system and oi a station of the system, respectively, but these figures do not pretend to give the circuit arrangen'ients since lin these figures single lines are used for the various transmission paths whereas in reality two or moriI conducting or control or other wires would be needed. ln the t'ollowing description Figs. 1 and 2 may, on account of'A their outline nature, conveniently be consulted where refereiiee is made to the general arrangcnient of :i system or ot' an individual station, but Figs. l to S, arranged as shown in Fig. 9, are relied on to show specific circuit arrangements; and wherever mention is made specifically of the circuit structures, reference should bc had to Figs. 8 toS tor the correspon(ling,r illustration.

Referring now to thc branches of flicy rccciving antenna system (hat constitute the incoming channels from the various distant stations, each ot these is connected serially with the output circuit ofi the receiving antenna balanced amplifier through thc medium of a timed circuit which provided with an anti-resonant shunt to aid in the diversion into the particular timed circuit of thc frcqneneies lo which i'he circuit is tuned and the unattcnuated passage through the series circuit of the frequencies lo which thc other timed. branches are responsive. For a more complete description of the action of these selecting circuits, reference may bc had to the patent to Raymond A. Helsing 1,313,483, August 19, 1919, in which similar selecting circuits are shown at a transmitting station. The spacing of the carrier waves is so close that the tuned circuitof each branch is not relied upon to effect in itself; the exclusion ot adjacent frequencies from the branch. but merely to effect as great a measure as possible of attenuation for all frequencies other than that for which the particular branch in question is timed. The manner in which the sclectivity of the channel for its particular frequency is increased and response to other frequencies prevented will be hereinafter dcscribed. The frequencies diverted from the common circuit by way of a tuned circuit are amplified and then passed to a balanced detector where they are combined with a locally generated wave developed in such a manner as to have identically the same frequency value as that ol the modulated carrier that is received over this timed branch. Among the demodulation products developed in the balanced detector as ay result of the combination of these frequencies are the reproduced low voice or signaling frequencies with which the carrier Wave was originally modulated at the fifi transmitting station. These low Voice frequencies, or signal controlling frequencies in the case of the control channel, are segregated from the other demodulation products by the intcrposition of a low pass filter in the output circuit of the balanced detector. The roicc or low frequency currents are then amplified and passed directly to the springj ack which constitutes the incoming terminal of this channel.

ln the case of the control channel. incoming from cach of theremote stations, the amplilied low frequency products of demodulation which are obtained through the operation of a series of elements `similar to thosevmentioned above, may include one or more of a number of lon7 frequency controlling Waves With which the carrier Wave is modulated at the transmitting station in accordance With the particular associated communicating channel that is to be used in establishing the connection and the particular station that is being called. onsequently the outputcircuit ot the last an'iplifier in the control channel is provided With a number of branches each of Which is selective of a particular one of the controlling lou7 frequencies with which the controlling` carrier Wave may be .modulatei ln the case of station 1 illustrated in Fig. 2 there are four selective branches of the control channel incoming from each of the distant stations, these selective branches be ing tuned to respond to 200 cycles, 1,000 cycles, 1,200 cycles and 1,400 cycles respectively. it the other-stations of the system, as may be seen by reference to Fig. 1 the tuned branches of each receivingv control channel are adapted to be selective of the same low control frequencies of 1,000 cycles, 1,200 cycles and 1,400 cycles, respectively, but the remaining` branch arranged to be selective of`400 cycles in the case of station 2, of 600 cycles in the case of station 3, and of 800 cycles in the case of station 4. Thus station 1 responds to the modulation of any of the control channel ari-ier waves with 200 cycles, station 2 responds to the modulation of a control channel Acarrier ware with 400cycles, station l to its modulation' with 600 cycles, and station 4 to its n'iodulation with S0() cycles. The

modulation ot' the corresponding control` channel carrier with 1,000 cycles occurs when a. transmitting station selects its transmitting' channel. 1 Jfor use, the n'iodulation oi the cor-- responding control cbannelcarrier With 1,200 cycles occurs When the transn'iitting station selects its channel 2 for use, and the mochila tion with 1,400 cycles occurs When the transmitting `station selects its channel 3 for use. lhe devices associated with the Various selective branches of each of the receiving` system control channels interact and cooperate in sucha Way that thelamp signal associated with a particular incoming communicating channel at a particular station is lighted when the transmitting station takes the correspond ing channel for use and signals the station in question.

As has been stated there are at each of the stations four transmitting channels, three of which terminate upon springjaclrs atthe operators position and are used for communieating purposes, and the fourth of Which is :n'rfmged to be used for calling and signal controllingpurposes. The Voice frequencies that are supplied to each of the communicating channels When connection is established by Way of the terminal springjack of the channel are carried through a lonT pass filter Which serves to suppress the upper harmonics and higher Voice frequencies and thus hold the frequencies Within a moderately restricted range. TheA voice frequencies that are conveyed through the lou7 pass filter are supplied to the input circuit of a balanced modulator Where they are combined with the appropriate one of the carrier Waves developed from the base frequencies as Will hereinafter be described. The products of modulation are then passed through a tuned circuit which serves to suppress all but the voice frequency modulations of the carrier Wave and such unmodulated component of the carrier Wave as is not balanced out and suppressed by the action of the balanced modulator. The voice modulated carrier then is amplified and passes into the common circuit of the transmitting antenna by Way of a tuned circuit With a shunt that is anti-resonant to the frequency of this particular transmitting` channel.

In the case of the fourth or control channel of the transmitting group the `input circuit of the balanced modulator is arranged to receive the locally generated carrier Wave which this channel employs, and also any one or more of the previously mentioned group of lorsT frequency controlling Waves. The controlling Waves are generated as harmonics of a base frequency or fundamental produced in an oscillator, this fundamental frequency in the present instance being-200 cycles. Harmonics or multiples of this frequency are produced in a harmonic generator, are then am: pliiied and distributed by means of tuned circuits to other amplifiers Where each harmonic that is to be used is amplified to Whatever degree may be desired, and is applied as required through a tuned branch to the input circuit of the control channel balanced modu later. In the case of station 1 illustrated in Fig. 2 the harmonics that are segregated and amplified for use are 400, 600, 800, 1,000, 1,200, and 1,400 cycles respectively. The 200 cycle frequency is not utilized at station 1 for the reason that this is the frequency with which the other distant stations modulate their controlling carrier Waves to select station 1. In like manner as indicated in Fig. 1 station 2 omits the 400 cycle harmonic by means of which lit isadapted to be selected by other stations, station 3 omits the 600 cycle harmonic andl station 4 omits the 800 cycle harmonic. Thus the Vcont-rol frequency generating apparatus of each station makes available for use all of the frequencies up to 1,400 cycles excepting the particular 'frequency or hai'- monic that is used in vetl'ecting the select-ion of this'wparticular station. -Such control frequency or combinationrof control frequencies asjmay be required in the establislnnent and.

control of connections are made effective by meansof relays that control the energizing circuits of the amplifiers in the corresponding tunedbrauches of the harmonic distributing eir'cuit.' -The three 'relays controlling the 1,000, y1,200 and 1,400 cycle branches, used to identify the particular' transmitting chan- Vwill be keys K-l, K-B and iK-/t (not shown) at station 2, keys K-l, K-2 and K-/ (not'nshoivn) at station 3, and keys K-l, K-j-2 and .K-B (not shown), at station 4. j Y

As shown in Figj2, the operators position is providedwith three connecting circuits forestablishing a tivo-way radio link with any one of the distant stations and for bringin'g this two-Way link into connection with the lines local to the associated station. These connecting circuits are each provided with a receiving,r plug adapted for insertion in any one4 of the springjack terminals of the nine vchannels incoming from the three distant stations, a transmitting plug for insertion in any one of the three springja-ek terminals of the gthree communication transmitting channels of the associated station, and a local plug for insertion in the springj ack of any one of the local lines. Each connecting circuit includes the usual hybrid coil and balancing network for rendering the receivingr and transmitting branches of the circuit conJugate .with respect to each other. The connecting circuits also have connection with the usual operators telephone set and ringing generator by means of the customary listening and ringingkeys, and in addition are provided with controlling keys for governing the application of the desired loivfrequency controlling currents for 'the modulation of the associated controlling channel, (see Figs. 5 and 6) These parts with their associated controlled and controlling elementsv and circuits Will be described in connection With the detailed description of at this point that each connecting circuit has I associated with it asupervisory lamp that is under the control of the distant station through the medium of the corresponding receiving plug and receiving channel springjack and the controlling channel incoming `from the distant station.

Referring now to the apparatus for developing the high frequency carrier and demodulating waves from the received hase frequencies, the base frequencies of 100,000 and 107,000 cycles after passing through the hund tilter shown in the lower left hand corner ot Fig. 2 pass to the input circuit of the detector. From the output circuit of the detector the base frequency of 100,000 cycles and the difference frequency of 7,000 cyclesare separated out by means of tuned circuits, and each of these frequencies after :unplifieation is caused to produce harmonies by means of harmonic generators included in each circuit. After amplification the 600,000 and 700,000 cycle harmonics of the 100,000 cycle hase are selected and pass into a common distributing circuit. In like manner the harN monies developed in the harmonic generator ol' the 7,000 cycle circuit are amplified and the 11i-,000 cycle, 21,000 cycle, 28,000 cycle, and 35,000 cycle harmonics are then separated out by means of tuned circuits. Each of the last mentioned harmonics of the 7,000 cycle fundamental is then separately amplified and passes t0 the input circuit of a mod-- ulator Where in each case it is combined with amplified 600,000 cycle and 700,000 cycle frequencies derived from the common distributing circuit. As a result there is presentin the output circuit ot the lower one of the four modulators the upper and lower side bands of 600,000 modulated by 14,000 and the upper and lower side bands of 700,000 modulated by 14,000; that is. the frequencies 586,000, 614,000, 686,000 and 714,000. In like manner the modulation of 600,000 and 700,000 by means of the 21,000 frequency in the seeond modulator produces the frequencies 579,000, 621,000, 679,000 and 721,000. In the third modulator the frequencies 572,000. 628,000, 672,000 and 728,000 are produced; and in the fourth or upper modulator the frequencies 565,000, 635,000, 665,000 and 735,000 are produced. There are also, of course, other products of modulation in the various output circuits; but they are widely separated in the frequency range from the groups of frequencies above mentioned, so that they are readily suppressed in the tuned circuits that take off for use from the outputcireuits the frequencies mentioned.

It will be noted that the frequencies specifically mentioned as the combination products of the harmonics developed from the two hase frequencies are the same frequencies as those which have heretofore been mentioned as being employed for demodulation moaoafi of the incoming inodulated carriers ie thereceiving channels and for modulation by the 'voice and lower controlling frequencies in the transn'iitting channels. The useful freqiiencies present iu the output circuits of the four harmonic modulalors are derived from the respective circuits by means of tuned circuits at the various points `c-:here they' are to be used and are applied to the input circuits of the balanced detectors and the balanced inodulators of the various channels through-the niediuin of amplifiers connected with` the tuned circuits.

Reference will now be had more part-icularly to Figs. 3 to 8 inclusive of the drawing, which illustrate as much of the preferred circuit organization of a single one of the stations of the system as is necessary to enable the invention to be understood. Before proceeding to a detailed description of the operation of the system, however, the orzjanization and operation of that part of the system which serves to generate, modulate, transmit, receive and detect or deinodulate the high frequency carrier waves will be described in sufficient particularity so that a d ctailed reference to the functions and Inode of operation of these elements will not be needed in the description of the operation of the system as a whole which follows:

The receiving antenna s-aysteni at each station includes` a receiving antenna RA connected by way of a band `filter Blf-1, with one winding of au inductive coupling l01 the other winding of which is connected with the input circuit of a balanced amplifier 13A-1. The baud filter Blf-1 is of the general type disclosed in the U. S. Patents to Geo. A. Campbell, Numbers 1,227,113 and 1,227,114; issued May 22, 1917. This band filter is designed in accordance with the principles of the said Campbell patents to shunt or divert from the circuit of the receivingl antenna allv frequencies in. the bands of frequencies that are adapted to be propagated from the transmitting antenna. TA of the corresponding station, (Fig. il). and to permit to flow rially iu the receiving antenna circuit without substantial attenuation all frequencies` that are adapted to lit-e received freni the distant stat-ions. "llo assist in dissipating the energy of the frequencies diverted through the band filter Blf-*1, the band filter is connected with an energy dissiiniting device E02 which may take the ferne. of :i rertance of such value as te perform the funcl `on stated without interfering witlithe selective characteristics of the band filter FiF-1. The remaining frequencies, those which are not blocked or trapi'ied by the band filter liliL-l, act inductively through the inediuin of the windings of the inductive, coupling 101. to control theoperation of the balanced amplifier BA-L This amplifier' is of the well known the push-pull type, and serves as a repeater h ving in effect a substantially linear characteristic throughout a large operating range to permit of the repeating of the carriers that pass through it without intermodulation betwecn theni. A luilanced amplifier or repeater arranged to function in this nuinucr is disclosed in U. pet-eu to li. W. Kendalh N o. 1,5/ Ll,910, dated July 7, 1925.

After amplification in the balanced amplifier or repeater Br M1 the received carrier and base frequency waves pass by wayof the common' circuit 103 to a plurality of tuned branches where the different frequencies are selected and diverted into their separate channels. Referring first to the amplified base frequencies, which in the present keinbodiinent of the invention are assigned the values of 100,000 and 107,000 cycles per second, these frequencies are selected from the coininon circuit 103 by way of the ln'anch including the band filter :BF-2. This filter is of the Campbell band tilte ype previously' referred to, and is so constructed as to divert 100,000 and 107,000 frequencies with substantially negligible atteniuttion and to prevent the entrance into the correslviondingbranch of the other frequencies present in the coininon circuit. The base frequencies that flow through the band filter BF-Q pass to a detector DH1 from the output circuit of which the base frequency of 100,000 cycles lected by means `of a tuned circuit 'fC- 8, while the difference frequency of 7 ,000 resultin g from the deinodulation of the 100,000 and 107,000 frequencies is selected by ineens of a tuned circuit rITC-9. The base freq ueney of 100,000 thus selected is amplified by the amplifier ,ifi-8 and the amplified oscillations rare impressed upon the input circuit of the tl'ierinionic tube TIG-8. rllhe magnitude of the oscillations is such to overload this tube, with the result that, as disclosed in U. S. patent to ll. lill. Kendall, No. 1,4lll0 752 dated February 27, 19:23, the repeated oscillations are distorted into a series of currents the frequencies of which are the harrnoi'iics ot the base frequency current, the tube HGr-8 thus acting as a harmonic generator. rllhe base frequency and its harmonics are then amplified by the amplifier fin-01 and the sixth and seventh lmriuonics, ($00,000 cycles and 700,000 cycles, respectively, pass by wav of the trans'hn'iuer 105, are selected by the tuned circuits ',lC--G and 'ri`{l--7 and are iinpl'es, Hyd upon the distrilalting circuit 10i.

In like manner the 7,000 cycle freqi'lency se lccted by the tuned circuit TC--9 is amplified by the aniplifiei 9 and the amplified oscillations are impressed upon the input circuit of the harmonic generator' HGM-0 where they are distorted into a series of currents the frequencies of which arethe harmonics of the base frequency of 7,000 cycles. The base frequency audits harmonics are thea amplified by the amplifier .11H91 and are impi'essed by Way of the' windings of the transformer 106 upoii a distributing circuit 107.

The twoharmonics of the base frequency 100,000 that are present in distributing circuit 104 and the four harmonics of the derived base frequency 7,000 that are present y fin the distributing circuit 107 are coml'iined in a manner that will now specifically be described to produce sixteen carrier and de modulating frequencies which are conveniently generated in four `groups of four frequencies in each group. As the selection of a ple tuned circuits. To this end each of the harmonics of the lower base 7,000 is conibined with the two harmonics of the higher base 100,000 in a modulator separate from that in which each other harmonic of the lower base is combined with the two harmonics of the higher bases. rlhus the combination frequencies produced in the output circuits kof the various modulators are separated from each other by a greater frequency interval 'than would be the case if the harmonies of the lower base werecombinedA with the harmonies of the higher basel in a common corn-V bining or modulating device, and the combination frequencies in the output circuits are therefore far enough apart to permit of their ready separation by means of ordinary resonance devices. l

A description of one of the' harmonic frequency combining ciicuits will serve for all of them, as theyvare a'll alike excepting with respect to the frequencies that are supplied to them from the various branches Vof the distributing circuit 107 of-the lower base frequency harmonics. From the distributing circuit 107 the 14,000 cycle haimonic of the 7,000 cycle base is taken off by means of a tuned circuit rl`C-2 and is applied to the terminals of a potentiometer 108 by means of which such'fraction of the oscillatory current potential as may be desired is applied to the input circuit of amplifier 4 2. The output circuit of this amplifier is connected through a potentiometer 109l with the input circuit ofthe modulator M-1. The circuit arrangement of this modulator as shown is of the type shown in the United States patent to Van der Bijl 1,350,7 52 dated August 24, 1920. For a discussion of the action of a modulator in producing the combinationfrequencies of the impressed Waves, the British Patent No. 102,- 508 may be referred to, or the Colpitts and Blackwell publication above cited. At the saine time the 600,000 and 700,000 cycle frequencies present in the distributing circuit 104 are applied by way of a potentiometer 110 to the input circuit of an amplifier A-2, and the output circuit of this amplifier is connected by way of potentiometer 111 and inductive coupling 112 with the input circuit of the same modulator M--l with which the. 14,000 cycle frequency from the distributing circuit 107 has been described as being connected. As a result there are present in the output circuit of the modulator M--1 the sum and difference frequencies of each of the higher frequencies 000,000 and 700,000 with the lower frequency 14,000. These frequencies are applied to the distributing circuit 113 by way of the windings of a coupling coil or transformer 117. In like manner the 21,000 frequency is selected from distributing circuit 107 by way of tuned circuit TC-3, amplified by amplifier A- and is applied to the input circuit of modulator M-2 where it is joined by the 600,000 and 700,000 frequencies that are applied to the input circuit of modulator MM2 by way of amplifier A-Bl; and these frequencies are combined to produce sum and difference frequencies of the lower frequency 21,000 with each of the higher frequencies 600,000 and 700,000, which frequencies are supplied to the dis tributing circuit 114. Similarly they 28,000 lower harmonic frequency is su plied to the input circuit of modulator 3 by way of tuned circuit 'fC-4 and amplifier A4 and is there joined by the two higher harmonic frequencies supplied by way of amplifier A-41, and corresponding sum and difference frequencies are created in the output circuit of this amplifier, and are supplied to distributing circuit 115. Likewise the lower harmonic frequency 35,000 is supplied to the input circuit of modulator M-4 by way of timed circuit 'FC-5 and amplifier A-5, is joined by the two higher harmonie frequencies supplied by way of amplifier A-5, and corresponding sum and difference frequencies appear in the output circuit of modulator M-4 and are supplied to distributing circuit 116. As a result there are present in the distributing circuit 113 the frequencies 566,000, 614,000, 686,000 and 714,000; in dun tributing circuit 114, the frequencies 57 9,000, 621,000, 679,000 and. 721,000; in distributing circuit 115, the frequencies 572,000, 628,000, 672,000 and 728,000; and in distributing circuit 116, the frequencies 565,000, 635,000, 665.- 000 and 735,000. It may be seen that the fm'- quencies in each of these distributing circuits are sufficiently spaced apart so that they are readily capable of clean-cut segregation by means of ordinary tuned circuits. Of course there are present in each of the distributin circuits the unmodulated components as we as the sum and difference frequencies of the two higher component frequencies 600,000 and 700,000. But all of these frequencies are sufficiently widely separated inthe frequency spectrum from the frequencies that are to be derived from the distributing circuits as carrier and demodulating waves to prevent them from passing through the` tuned branches through which the useful frequencies are taken off.

Of the four modulatingl or frequencycombining` branches, the three which feed distributincircuits 113, 114i and 115 are controlled by relays 13, 14 and 15 respectively and are normally inert. The fourth branch, that which fcedsthe distributing; circuit 110, is

permanently energized, as it is the frequencies produced in this branch that are supplied to the calling` or controlling receiving chair nels, which channels are maintained in a non mally responsive condition.

Of course it will be understood that the carrier and demodulating wave distributing;` circuits 113, 114 and 115 and 11G are to be so located and shielded from each othe and the other circuits of the system as to prevent any objectionable interference and interaction arising from the hi gli frequency currents that they carry as well as excessive attenuation of such currents. Ways in which this result may be accomplished are well known in the art.

Specific reference will now be made to the organization and operation of one of the cominunication receiving channels associated with the receiving; antenna system. The receivingl channels RC-413 will be described, and it will be understood that the other communication receiving; channels are similar to this excepting in the matter of the particular frequen cies to which they are adapted to be selective. The receiving branch RC-43 has connection with the cominoncircuit 103 of the receiving antenna system by way of the shunted tuned circuit STC-43. The shunted tuned circuit STC-*43 includes a series resonant circuit with an anti-resonant shunt in the common circuit 103, this shunt being' anti-resonant to the same frequency to which the series resonant circuit is tuned. Consequently the particular carrier frequency to which this receiving' branch is adapted to respond is caused to pass through the primary winding of the coupling coil that forms part of the series resonant circuit, While the other frequencies are provided with a path of low attenuation by way of the antiresonant shunt. The ondary winding` of the coupling;1 coil is inn eluded in a circuit resonant to the frequency in question, and the carrier wave of correspending' frequency thus diverted from the common circuit is applied by way of a potentiometer 118 to the input circuit of an annili iier A-J13. The output circuit of the amplifier is connected by way of a potentiometer 119 and a transformer 120 with the two input circuits of a balanced detector B13-i3. Associated with the communication receiving branch lC--lll there is a tuned circuit Til-413 that has connection with the distributing circuit 115 of the carrier frequency gen erating apparatus. The constants of this tuned circuit are such as to cause it to take off from the distributing circuit 115 the 728, 000 cycle frequency, which is the same frequency as that to which the tuned circuit SiC-413 is arranged to be responsive. The 723,000 cycle frequency that is diverted by the tuned circuit 'fC-113 is applied by way of a potentiometer 121 to the input circuit of au'iplifier lf-44u31, and the output circuitv of this amplifier is connected to the common conductor of the two input circui s of the balanced detector 13D-13 by way of a potention'ieter 122.. pla-ined and as is disclosed in British Patent No. 102,503 above referred to, and in United States patent to Carson 1,343,306,

il une 15, 1920, the carrier wave at the transmitting station modulated by the speech or voice currents in a modulator of a type that operates to suppress the unmodulated component of the carrier and transmit 1nere ly the speech modulated side bands. As the frequency of J@3,000 that is applied to the balanced detector B13-43 by way of tuned circuit 'IC-113, amplifier A-4L31 and the potentioineter 122 in the common conductor of the input circuit corresponds to the carrier frequency that was" suppressed at the transmittingi1 station, the detector acts, in aceordance with the principle disclosed in U. S. Patent 1,313,300, to Carson, and in British Patent No. 102,503, above cited, to demodulate the voice modulated carrier side bands and immediately reproduce the voice currents in the output circuit of the modulator. The output circuit of modulator BDA?) .is connected by way of the windings of transformer 123 with the low pass filter LPF-413- The constants of this filter, which is constructed in accordance with the principles disclosed in the Campbell patents hereinbefore referred to, are such that. all of the hi lq'licr frequencies resulting from demodulation are sumnessed, and only the voice frequencies reproduced in the output circuit of the modulator are permitted to pass. The voice currents that are carried through the low pass 'filter LPF i3 are applied through the mediuin of the transformer 1211 to the inputcircuit of the amplifier A--l132 and the output circuit of this amplifier is connected by way of the windings of transformer 125 with the line contacts of the receiving sprinpgjaclr liti-13 associated with this particular receiving channel.. The conductors over which battei y current supplied for the energization of the therinionic tubes in the conimunication receiving); channel liti-113 are controlled in contacts of a relay 16. These contacts are As will hereinafter be cnf normally open so that the tubes of the receiving channel are normally inert.

V-As has been statcichthe other communication Y receiving channels are organized in ex- Cir l actly the saine manner as channel RC Y which has `iust been descriocd, excepting that ving channel ,RC-13.

, i) elo,

theconstants of their associated tuned circuits are so adjusted as to divert from the connnon receiving autennacircuit 103 the particularl modulated carrier waves to which the channels arevrespeclively adapted to respond, and to select from the distributing circuits of the carrier frequency generating Aapparatus the particular coiresponding frequencies that are required for the demodulation of the modulated carriers that enter the associated channels. The voice currents that are reproduced by the-demodulating action inchannel RCV-4l are conducted to the line contacts of receivingl springjack ltd-41, and

the Voice currents reproduced-.by the deinodadapted to respond is diverted from the coin-V inon receiving antenna circuit 103 by way of the anti-resonant shunted tuned circuit STC-4l and' isiinpressed by way of a potentiometer upon'the input circuit `of the ainplitierA-llll, The current produced in the, output circuit of this amplifier is impressed through a potentiometer upon the two input circuits of the balanced detector BD--fll through the medium of a transformer. At the same time the correspond-V y ing frequency is taken from distributing circuit -116 of the carrier frequency generating apparatus by way of tuned circiut 'ITC-44 and after amplification in amplifier A-Ml 'is applied to the common con-ductor of the two input'branchcs of balanced detector BDMl-L As in the case of receiving channel BCV-48, the demodulating action which occurs in the balanced detector 13D-44 is such as immediately to reproduce in the output circuit of the balanced detector the particular controlling frequencies with which the carrier Waves of controlling channel BiC-lill was modulated at the transmitting station. The'low pass filter LPF-#f4 acts to'supprcss all of the higher frequency products of demodulation and to impress the reproduced low frequency controlling currents upon the input circuit of amplifier A-4/l2.

The output circuit of amplifier lli-442 is connected by way of the windings of transformer 126 with a distrilmtingr circuit 127. Connected with distributing circuit 127 are a inunber of branches Y128, 129, 130 and 131 each nnnle resonant to a different. one of the cont-rolling frequencies by the tuning derespectively. ln each of the, tuned circuits, and constituting a part ef the inductance, is a relay so constructed and adjusted that it maintains a contact normally closed, but when traversed by alternating current of the frequency to which the associated branch is responsive, it holds its contact open. These relays in the four-tuned branches are designated respectively 18, 19,20 and 2l.. The last mentioned relays cooperate with corresponding` associated relays Q2, 23, 2l and 25; and the relays 23, Q4 and .25 have operatively associated with them corresponding relays 26, Q7 and Q8. The manner in which the above mentioned relays, together with relays 29, 30 and 3l, associated. respectively with the terminals of the communication receiving channels llC-4L RCM-l2, and RC-ll, cooperate in controlling the display of the lille signal lamps L-;'l, L42 and L4l of the three communication receiving channels respectively will be explained herein- `after in connection with the description of the operation of the system.

Reference will now be made to the transmitting channels for modulatingr the high frequency carrier waves assigned for transmission purposes to this Istation with the low frequency voice and c"introllingl currents. In Fig. 8 of the detail drawing these channels are designated C-ll, CM1?, Cl3 and C-ll. The first three arey the conin'lunication transmitting channels and the last one is the calling or controllingr transmitting channel. lnasniuch as the communication transmitting channels are substantially identical in their organization in all respects excepting in the matter of the value of the high frequencies that they are adapted to control, the organization of channel (l---l-l is the onlyone of the communication transmitting channels that is shown in detail, the other channels being' rimresented merely by squares bounded by brolten lines. As the organisation of the controlling transmission channel CMM dilters slightly from that of the communicating transmission channels, it also is illustrated in detail.

The line contacts of the springjack 'FJ-3 in which the communication transmitting channel C-13 terminates, are connected by Way of a low pass filter LPF-13 and a potentiometer 132 with the primary windingy of a. transformer 133. The low pass filter LPF-13 is constructed in accordance with the principles of the Campbell patents hereinbefore referred to. and its constants are so chosen and adjusted that it passes with subvi'ously described, is adapted to pass'with slight'and substantially negligible attenuation the frequencies in the side bands result-- ing :troni the modulation ot thevcarrier wave without distortion; oscillations in the mitputcn'cmt ot the bai-- with the relatively low cont-rol frequencies.

The tuned circuit STC-14 is resonant to thisnarrew bandedl trequcneies; and the presence' otthe anti-resonant shunt comprising ,partof the element STC-14 prevents these frequency bands from bein g shunted, but permits the serial passage through the antenna nag conductor ofthe frequency bands produced by Vinodulation in the other transmitting channels .i Y The supply' of vcurrent tor'energizing the ythermionic tubes ot the control transmitting channel C14is controlled in contacts of relay 32. These contacts are normally open, and therefore the control transmitting channel is normally inert. ln like manner the supply of battery current tor energizing com- Amunication transmitting channel C-13 is controlled in normally open contacts of relay 33, and other similar relays control the energization of the other eoimnunication transmitting channels C-11 and @-12 in like Referring now to the organization by means ol' which the lower cont ol frequencies are generated, as shown in Fig. S, these y frequencies are derived as multiples or harmonics o'll fundamental or base frequency generated by the oscillator 0 1. This oscillator may be ot any usual and well known t-herniiomc tty-'pe with a teed back connection ',l, v '1 7 irom ns output into its input circuit. lhe

, frequency otthe oscillator current `productal monic generator HGr-1 are ot non-sinusoidal wave torni, which is the equivalent ot a base Afrequency current ot sinusoidal form and a series oli currents of frequencies which are .multiples or liarnioni s or" the base lires queney. vThe ae.1on is the saine as that previously described in connection with the harmonic generators illustrated in Fig. 5 ot theV drawing. The harmonic` frequencies produced by the harmonic generator HGr-1 are ampli lied in the ampliiier A-l and are then supplied to the distributing circuit 145 by way otl the windings of a transformer 146. From the distributing circuit 145 the second, third, th and seventh harmonics L" means o1' the tuned circuits are rselected 'PG-52, 'fC-53, TC54, T (lv-61, TC-62 and 'fC-03 respectively. Each of thc harmonics thus selected is supplied through a potentiometer to the input circuit of the corresponding series ot ampliliers A--52, A53, ill-54. A-01, A-GQ and A-63 respectively. The output circuits of these amplilicrs are respectively connected by wa ot the tuning devices TCP-f2', 'fC-53, 'EC-5ft', TC*61, 'fC-(32 and 'PCs-63 with the common collectingr circuit 147, and the circuit 1-17 is connected by way ot the windings ot the transformer 118 with the 'ii-cuit 138 which. has previously been dc- :fcribed as supplying the control modulations to 'the control transmission channel C-14. lhe tretuiencies ot 400, 600, 800, 1,000, 1,200 and 1,100 cycles per second are thus produced and made available tor application to the modulatingl circuits ot the control transmitting channel C-LL The constants of the timing devices in the respective. branches are ot course adjusted to pass the particular frequency that cach corresponding branch is intended to pass and (o bar ont the other frequencies. 'l`he supply of battery current for energizing the tubos of ampliiers ifi-52, A-, cte., is controlled in the normally open contacts of a series of rclays 52, 53, 5-1, 01, 02 and (i3 associated rcspectively with the corrcspomling amplifiers. 'lhe manner vin which these relays are controlled to bring about thc encrgization of their respectively associated amplilicrs will be described hereinafter.

rlhe specilie organization ot the portions ot thesystem that have not thus far been particularly explained will best bc understood in connection with the detailed description of the operation ot the system which follows.

le will assume that the operator at station 1 wishes to get into connuuuicatiou with the operator at one ot the other radio stations or centers in order to establish telephonie connection between one ot the local lines of station 1 and one ot the .local lines of the other station. For initiating the call there are available to the operator at the calling station three comumnication transmitting channels which terminate respectively upon the springjacks Tflwl, TJ-2 and 'FJ- 3. The calling operator also has available 'l or eskilishing the connection the three connect- 'ng circuit-s @fC-1, CC--Q and CC-3. each ot' which has a plug lor connecting with thc local line and two other plugs for connecting respectively with the receiving and transi'nitting channels that are temporarily associated to constitute the two-way radio linlr.

Noting that one of the communication transmitting channels, say the channel terminating in springjaek 'TJ-3, is available for use, and that one of the connecting circuits, say the connecting circuit CCQ-S, is also avail- 

